Electron emission control



June 3, 1969 w. c. NIXON 3,443,327

ELECTRON EMISSION CONTROL Filed June a, 1967 Sheet of 2 June 3, 1969 w. c. NIXON 3,443,327

ELECTRON EMISSION CONTROL Filed June a, 1967 Sheet 2 of 2 WM .BY Algal U4 5 11021 [mm a United States Patent US. Cl. 315-48 Claims ABSTRACT OF THE DISCLOSURE The specification discloses an arrangement for producing an electron beam in which the energies of individual electrons are restricted to within predetermined limits. This is done by filtering unwanted electrons from the beam in two stages, more specifically by diverting the beam to a branch path where all electrons but those having energies above the upper predetermined limit are reflected and are in turn deflected to an exit path where a second filter reflects all electrons having energies below the lower predetermined limit. The specification further describes how the rejected electrons can be employed to stabilize the deflection of the electron beam.

This invention relates to the control of electron emis- 810115.

In electron optics, the control of the motion of a beam of electrons may be related by mathematical analogy to the control of light energy. Like a light beam, it is possible to focus an electron beam and the similarity in such respects as this extends to the problem of chromatic aberration. In an electron beam such aberration arises from the different energy levels that the individual electrons may have.

The present invention is concerned with a method of and apparatus for rendering an electron beam wholly or substantially monochromatic, that is to say, to produce a beam in which the various energy levels of the electrons in it are contained between predetermined upper and lower limits.

According to the method of the present invention for producing an electron beam having a restricted range of electron energy levels, the beam from an electron source is directed to a first filter means where the electrons are divided into two streams in which the electron energies are respectively above and below a first predetermined level, one of said streams being directed to second filter means where a similar separation is made in dependence upon a second predetermined energy level such that one of the divided streams from said second means has its electron energies limited to the range between said two levels.

ConvenientlyTne beam is deflected by a magnetic field in its progress from the source to a first, upper energy level filter means, the lower energy electrons reflected by the filter means being returned through the magnetic field which will give a further deflection of the beam to the axis of an exit path, second, lower energy level filter means in the exit path allowing through only those electrons of the reflected beam having energy levels above a predetermined minimum. The resultant beam therefore has its energy levels restricted between the two values set by the filter means.

Moreover, if it is arranged that the magnetic field deflects the beams through 90 at each passage of a beam therethrough, the resultant emission from the second filter means can be directed along an extension of the original beam path.

Monochromator apparatus according to the invention for use with an electron beam source may comprise means arranged to produce a magnetic field directed transversely to an electron flow path from said source to deflect the electrons from said path to a branch path in which is arranged first electron mirror means to reflect all electrons having less than a first, upper predetermmined energy level back along said branch path to the magnetic field, an exit path for the reflected electrons after redeflection by the field having arranged in it second electron mirror means to reflect all electrons having less than a second, lower predetermined energy level, the values of said first and second energy levels thereby determining the degree of monochromatism of an electron beam passing through the second mirror means.

The invention will now be more particularly described with reference to the accopmanying drawings in which:

FIG. '1 is a diagrammatic view of a monochromator according to the invention,

FIG. 2 is a sectional elevation of a constructional example of the invention showing the monochromator body, and

FIG. 3 is a composite view of the construction in FIG. 2, the left-hand half being taken at right angles to the plane of the section in FIG. 2 and the right-hand half being in an axial plane at 45 to the planes of the other two sections.

Referring more particularly to FIG. 1 of the drawings, an electron source 2 is shown, the emission from which is focussed, if necessary, by an electron lens system 4, which may be of electrostatic or electromagnetic form, into a beam that is projected towards a magnetic field 8 directed perpendicularly to the beam and the plane of the illustration. The field 8 deflects the beam to a path 10 and the strength of the field is preferably so chosen that the angle of deflection is In the path 10 is an electron mirror 12 held at a negative potential relative to the electron source. The potential diflerence is so chosen that not all the electrons may be reflected at the mirror, that is to say, those electrons having more than a predetermined energy level will pass through the mirror.

The electrons reflected from the mirror 12 return along the path 10 to the magnetic field '8 where, of course, they are deflected again through a further 90, the beam now travelling along path 14 co-axially to the original path from the source 2. In the exit path 14 is a second electron mirror 16 held at a potential that is negative to the electron source but less so than the mirror 12. The mirror 16 thereby reflects only those of the electrons which have energy levels below a second predetermined value lower than the first energy level cut-otf associated with the mirror 12.

Electrons passing through the mirror 16 therefore form an exit beam having electron energy levels limited to between two predetermined boundary values represented by the cutoff values of the first and second mirrors, all other electrons in the emission spectrum of the source 2 having been filtered from the beam by the mirrors.

The high energy electrons penetrating the first mirror 12 may be used, if desired, in the measurement and/or control of the magnetic field so as to stabilise the path of the exit beam. Thus, a fluorescent screen 20 may be placed beyond the mirror 12 in the path of the high energy electrons and the position at which they strike the screen will indicate the angle through which the beam 4 has been deflected. As an alternative to this, a form of automatic control is shown in FIG. 1, this comprising laterally spaced collector plates 22 connected to a balance unit 24 which controls one of the input conditions determining the deflection of the beam. As illustrated, the unit 24 is connected to the supply to coils 26 of an electromagnet producing the field 8. Thus, depending upon which collector plate receives the electrons, the balance unit is arranged to adjust the coil current either above or below a given mean value to bring the electron path back to a position between the plates 22. Such control could also operate by adjusting the voltage of the electron source 2 instead of current in the coils 26.

In the example shown in FIGS. 2 and 3, an electron gun of conventional form is connected to a mounting 32 of a tubular shroud 34 in which the gun is radially adjustable by screws such as 36. A conventional specimen chamber 38 in which the beam target is to be placed is connected to the shroud 34 through an interposed tubular section 40 that houses elements of the monochromator, a main part of the assembly there being carried by a flange plate 42 sandwiched between the section 40 and carrier plate 39 of the chamber 38.

The plate 42 has secured to it an insulator body 44 with a central cylindrical passage 46 and a branch cylindrical passage 48 extending to one side of the passage 46. Mounted in its holder 50 at the radially outer end of the branch passage by a retaining plug tube 52 is a first electron mirror 54. Similarly mounted through holder 56 and .plug tube 58 at the exit end of the passage 46 is a second electron mirror 60. It will be noted that the tubes 52, 58 are in electrical contact with their mirrors. The extended form of the tube 58 therefore defines a uni-potential field for the electrons transmitted through the mirror 60.

In contrast to the arrangement illustrated in FIG. 1, the deflecting magnetic field is provided in this example by a pair of co-axial, cylindrical bar permanent magnets 74, one of which can be seen in FIG. 3, joined by a yoke 75 spaced from the tubular section 40. Each magnet fits in a gas-tight cylindrical pocket 76 that is itself sealed to the tubular section 40 and that has its end resting in a recess 78 in the insulator body 44 adjacent the passage 46 at its junction with the branch passage 48. By virtue of this arrangement, the magnets are directly accessible for adjustment of their positions in these pockets to regulate the magnetic field between them.

For focussing of the emission from the gun 30, the insulator 44 also carries, at the inlet end of the passage 46, a focussing anode 62. A further focussing anode 64 is mounted beyond the insulator on plate 39 for the monochromatic electron beam emergent through the tube 58.

Since the interior of the apparatus is evacuated, the electrical connections to the mirrors 54, 60 and the anodes 62, 64 are led in through sealed guides 68 in an insulator 70 itself sealingly secured to the wall of the tubular section 40. Further guides 72 take the leads (not shown) through the insulator body 44.

Operation of the described arrangement is similar to the example in FIG. 1. An electron beam from the gun 30 enters the passage 46 after passing through anode 62 and is deflected by the field of the magnets 74, through 90, to the branch passage 48. These high energy electrons penetrating the mirror 54 strike a fluorescent screen in the tubular section 40 so that observation of their deflection and intensity is possible. The electrons reflected by the mirror 54 return to the magnetic field and by re-deflection continue along the passage 46 to the second electron mirror 60 where the second filtering process occurs and a monochromatic beam is directed co-axially to the gun 30, from the tube 58 through the anode 64 to the specimen chamber 38.

Automatic control of the deflection of the beam can be eflected by the use of a pair of collector plates and a balance unit similar to the items 22, 24 illustrated in FIG. 1. The plates would, of course, be located in the section 40 radially outwardly of the mirror 54 and its tube 52. Sealed leads through the insulator 70 would connect the plates to the balance unit and since the defleeting field is generated by permanent magnets, the beam deflection can conveniently be stabilised by using the balance unit to control the voltage supply to the electron gun 30. This is indicated in FIG. 1 by the signal line -82 from the unit 24 to electron source control unit 84.

What I claim and desire to secure by Letters Patent is:

1. A method of producing an electron beam with a restricted range of electron energy levels, comprising the steps of: directing the beam from an electron source to a magnetic field, deflecting said beam by means of said field to a first filter means, dividing the electron beam using said filter means into two streams in which the electron energies are respectively above and below the upper energy level of said restricted range, reflecting the stream having electron energy levels below said upper level from said first filter means to the magnetic field and deflecting said stream by means of said field, one of said streams being directed to a second filter means, dividing the electrons of said stream using said filter means into two streams in which the electron energies are respectively above and below the lower energy level of said restricted range whereby one of the divided strams from said second means has its electron energies limited to the range between said two levels, and measuring the path of higher energy level electrons emerging unreflected from the first filter means to effect control of the angle of deflection of the electron beam formed by said one divided stream of electrons.

2. Monochromator apparatus for use with an electron beam source comprising, in combination, magnetic field generating means disposed at or adjacent the electron flow path from said source to deflect the electrons from said path to a branch path, first electron mirror means in said branch path to reflect all electrons having less than a first, upper predetermined energy level, an exit path for said reflected electrons, second electron mirror means in said exit path to reflect all electrons having less than a second, lower predetermined energy level, the values of said first and second energy levels thereby determining the degree of monochromatism of the electron beam passing through the second mirror means.

3. Apparatus according to claim 2 wherein said first mirror means is arranged to return the electrons reflected therefrom along said branch path whereby said electrons are arranged to be redeflected by said magnetic field to an exit path containing the second electron mirror means.

4. Apparatus according to claim 3 wherein said branch path is directed perpendicular to said path from the source and said exit path is coaxial to the source path.

5. Apparatus according to claim 2 wherein a tubular element is disposed immediately downstream of the second electron mirror means co-axially to said exit path and is arranged to be held at the same potential as the mirror means in order to provide a focussing action on the electron beam passing through the mirror means.

6. Apparatus according to claim 2 wherein detection means are arranged in said branch path downstream of the first mirror means, said detection means being arranged to sense the path of travel of high energy electrons passing unreflected through said mirror means, signals input means actuated from said detection means being arranged to control and thereby stabilise the deflection of the electron beam at the magnetic field.

7. Apparatus according to claim 6 wherein said detection means comprises a fluorescent screen to provide a visual indication of the path of said electrons.

8. Apparatus according to claim 6 wherein said detection means comprises a pair of electron-sensitive ele- -ments spaced opposite sides of the desired path of the high energy electrons, control means receiving an input from said elements and being arranged to vary the angle of deflection of the electron beam in dependence upon 5 differences in the rate of electron reception at the respective elements.

9. Apparatus according to claim 2 wherein a pair of bar magnets are disposed on opposite sides of the electron path to produce the deflecting magnetic field, mounting means receiving said magnets permitting displacements of the magnets towards and away from said path to vary the angle of deflection imposed.

10. Apparatus according to claim 9 wherein a casing of the apparatus is provided with co-axial pockets f0rming said mounting means, said pockets being sealed from the casing interior and the magnets being externally accessible in the pockets for said adjustment.

6 References Cited UNITED STATES PATENTS RODNEY D. BENNETT, IR., Primaly Examiner.

10 CHARLES L. WHITHAM, Assistant Examiner.

US. Cl. X.R. 

